This invention relates to the utilization of the Von Karman vortex principle for determining the flow rate of fluid through piping. Briefly, the Von Karman principle involves the phenomenon that, upon insertion of a bluff body in the stream of fluid flow, a vortices profile is created. The vortices shed alternately at periodic intervals from opposite sides of the bluff body. This particular vortex profile is known as the Von Karman "vortex street". The frequency of the vortices shed by the bluff body is proportional to the velocity of the fluid, either liquid or gas, flowing through the piping. By varying the dimensions of the bluff body, the frequency of the vortex shedding will likewise vary. Each bluff body has a constant known as the K factor. Thus, one can readily determine the flow of fluid through piping by monitoring the frequency of the vortex shedding and having a known K factor for the bluff body.
Numerous devices have been designed for implementing the Von Karman vortex principle in measuring the flow of a fluid through piping. One type of vortex flow meter currently known is the thermal sensor or "hot wire". In this particular vortex flow meter, the voltage and frequency of the sensor element varies with respect to changes in cooling rate, resulting in the intermittent passage of the vortices across the bluff body. The thermal sensor vortex flow meter has several inherent disadvantages, the first being a poor signal to noise ratio. Additionally, the thermal sensors are extremely susceptible to damage by vibrations. Also, the sensors are easily coated when implemented for measuring the flow rate of a gas, thereby adversely affecting the meter's accuracy. Further, the accuracy of the sensor significantly decreases when foreign articles from a measured liquid build up on the sensor.
An alternative vortex flow meter implements a disc or shuttle-type sensing element associated with a magnetic pick-up. Pressure pluses created by the vortices in the "vortex street" vibrate the shuttle or disc which is monitored by the magnetic pick-up sensor. However, due to the significant amount of energy required to oscillate the disc or shuttle, the meter cannot be used to satisfactorily determine the flow rate of a gas or similar fluids whose vortices do not exert significant pressure on the sensing body.
A number of vortex flow meters implement piezoelectric crystals or ceramic materials as sensing units. A commonly known piezoelectric flow meter uses a vortex-generating plate integrally attached to a sensing plate. The sensing plate includes a liquid-fluid cavity bounded by a pair of flexible diaphragms welded to the side walls of the sensing plate. This design has a number of inherent disadvantages, namely, a marginal low end measurement capability. This inadequacy of the flow meter is largely a result of a substantial portion of the vortex signal dampened by the oil contained within the cavity. Additionally, the process of welding the diaphragms to the side plates creates discontinuities between the two elements which will ultimately affect the accuracy of the meter. Also, the welds have been readily susceptible to fatigue when the flow meter is used in harsh environments.
To overcome some of the disadvantages of the aforementioned piezoelectric flow meters, it has been known to position the piezoelectric sensors in abutting contact with the diaphragms. In this type of meter, a hole is bored entirely through the sensing plate of the bluff body. Each piezoelectric sensor is placed on a diaphragm and epoxyed into the sensing plate. In epoxying the diaphragms to the piezoelectric sensor and subsequently to the sensing plate, air voids are created between the piezoelectric sensors and the diaphragms. This will ultimately dampen the actual vortex signal and thus significantly reduce the accuracy of the flow meter. Further, with time and in harsh environments, the epoxy deteriorates, resulting in the diaphragm loosening or falling off, causing the destruction of the piezoelectric sensors. Finally, the discontinuities between the diaphragms and the sensing plate dampen the vortex signals transmitted to the piezoelectric sensor.
A further type of piezoelectric flow meter currently in use senses the stress of an element created by the "vortex street". This requires a tremendous amount of energy to be exerted on the element before the frequency of the vortices can be detected by the piezoelectric sensors. This type of flow meter can only be used with a minimum Reynolds number of 20,000.
As is readily seen from the aforementioned discussion, prior to the present invention the industry has been lacking in a vortex flow meter capable of detecting low magnitude forces imparted on a bluff body and determining the flow rate of fluid in harsh environments for any substantial period of time.